Frequently asked questions
Below you will find a list of frequently asked questions that has been
compiled from your user experience and feedback.
In case that you do not find an answer to your question on this page you
may post your question on the ctools@ irap. omp. eu mailing list.
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This page will be regularily updated based on the most frequent questions
asked.
Are ctools and GammaLib specific to CTA?
The short answer is: ctools yes, GammaLib no.
ctools have been specifically designed to provide the functionalities that
are required to analyse CTA data.
This does not mean that ctools can not be used for analysing data from
other Imaging Air Cherenkov Telescopes; the sole requirement is that the
data and response files are provided in a CTA compliant format.
However, ctools are not designed to analyse data from Fermi/LAT,
INTEGRAL, COMPTEL or other gamma-ray telescopes.
Nevertheless, some joint multi-telescope analysis is supported by ctools.
GammaLib, however, is a universal framework with a core that is completely
independent of any telescope.
Plugins are used to interface GammaLib with the data and response files of
specific telescopes.
For every new instrument that shall be supported a new plugin needs to be
developed.
So far, plugins exists for CTA (and Imaging Air Cherenkov Telescopes in
general), Fermi/LAT, and COMPTEL.
Some support for integration of any arbitrary multi-wavelength data
exists.
I want to upgrade ctools, do I have to upgrade GammaLib?
For the moment yes.
The GammaLib interface is still not fully stabilised, hence ctools is
continuously adapted to match the interface evolution
(see the Download
Should I used binned or unbinned analysis?
This depends on the amount of data you want to analyse and to some extent
on the question you want to answer with your analysis.
For an unbinned analysis, the computation time increases about linearly
with the number of events and hence with the duration of the observation,
while for binned analysis the computation time depends only on the number
of bins that is used.
Consequently, for short observation times (below 30 hours), unbinned
analysis is faster, while for longer times it is advantageous to use
a binned analysis.
If you go to very low energies, either make sure that you have enough
energy bins in a binned analysis to sample properly the strong drop in the
effective area towards small energies, or better, use unbinned analysis.
If you’d like to fit a Gaussian line to your data you may also prefer
unbinned over binned analysis, as the fine sampling required to resolve
the line may require a prohibitive large number of energy bins for a binned
analysis.
How precise are ctools?
The ctools and gammalib codes have a numerical accuracy of better than 1%.
This means that if you use ctools to determine for example the flux
received from a source or the spectral points of an SED, the relative
precision of the flux or the spectral points is better than 1%.
The same is true for spatial parameters, such as source position or
source extension.
For many cases the actual numerical precision is in fact much better
than 1%, but in any case, it should never be worse.
Note, however, that this does not imply that source parameters can be
determined with CTA with an accuracy of 1%. The accuracy depends in the
end on the precision to which the instrument response function is known,
which should be more in the 10% range.
How to get a ctools log file when running the tool from Python?
By default, tools and scripts run from Python will not generate a log file.
The reason for this is that Python scripts are often used to build ctools
analysis pipelines and workflows, and one generally does not want that such
a script pollutes the workspace with log files. You can however instruct a
ctool or cscript to generate a log file by invoking the logFileOpen()
>>> import ctools
>>> sim = ctools . ctobssim ()
>>> sim . logFileOpen ()
>>> sim . run ()